1. Field of the Invention
The present invention relates to a circuit for eliminating radiated electric field noise in a video appliance, and more particularly to a circuit for eliminating radiated electric field noise in a video appliance which can eliminate noise of the radiated electric field generated from the front face of a cathode ray tube (CRT).
2. Description of the Prior Art
In general, a video appliance employing a CRT emits electron beams through an electron gun in order to display images on the CRT screen. Magnetic field of a deflection yoke mounted on the neck of the CRT (not illustrated in the drawings) is utilized to control emitting direction of the electron beams for an appropriate positioning on the screen. If electric current is to be flown through a deflection yoke to generate such magnetic field, however, electric field is also generated. Accordingly, a great amount of electronic wave is always generated around the periphery of the monitor. As a consequence, a video appliance normally requires an apparatus for eliminating the electronic wave in a video appliance.
FIG. 1 shows a circuit for eliminating electronic wave in a video appliance.
Referring to FIG. 1, the circuit for eliminating electronic wave comprises: a horizontal driving pulse generation section 70 for generating a horizontal driving pulse signal according to the horizontal synchronous signal inputted from the PC (not illustrated in the drawing); a flyback transformer 72 for generating a high voltage according to a horizontal driving pulse signal of the horizontal driving pulse generation section 70, and supplying the high voltage to an anode of a CRT 71; an electro-magnetic wave offset section 74 for correcting waveform of the negative polarity pulse signal applied from the induction coil 73 integrally wound at a predetermined number with the flyback transformer 72; and a lead wire 75 for offsetting an electro-magnetic wave generated from the front face of the CRT 71 according to the negative polarity pulse signal of the offset section 74.
The lead wire 75 is wound at least once along the external periphery of the CRT, at which a bezzle section is positioned, with no closing loop being formed.
The horizontal driving pulse generation section 70 comprises a transistor Q switched according to the horizontal synchronous signal inputted from the PC, and a deflection coil DY generated from the neck of the CRT 71 for deflecting the electrons on the screen of the CRT 71.
A capacitor C1 and a diode D1 are connected in parallel between the collector terminal of the transistor Q and the deflection coil. A capacitor C2 is connected to the other end of the deflection coil DY. An anode of the CRT 71 is connected to the secondary primary winding of the flyback transformer 72 via a diode D2.
The electro-magnetic wave offset section 74 includes a resistor R1 and a resistor R2 connected to one end of the induction coil 73 for dividing the voltage to level-adjust the negative polarity pulse signal applied by the induction coil 73. A capacitor C3 for correcting the waveform of the negative polarity pulse signal and a resistor R3 are connected in parallel to the other end of the resistor R1. A capacitor C4 and a resistor R4 are connected between the resistor R3 and the lead wire 75 for eliminating spike-like noise of the negative polarity pulse signal.
The reference C.sub.HV denotes a capacitor.
The conventional circuit for eliminating electronic wave in a video appliance operates as follows.
If a power supply is applied to a primary winding of the flyback transformer 72, and a horizontal synchronous signal is inputted to the base terminal of the transistor Q of the horizontal driving pulse generation section 70 from the PC, the transistor Q1 drives the flyback transformer 72. In other words, if the transistor Q is switched on/off, the flyback transformer 72 induces and applies the high voltage to the anode of the CRT 71.
The voltage at the primary winding of the flyback transformer 72 is inputted to the deflection coil DY as the transistor Q is switched on. Accordingly, deflected electric current flows through the deflection coil DY to deflect the electron beams emitted through the electron gun.
The negative polarity pulse voltage Va induced at the secondary winding of the flyback transformer 72, as expressed in the following equation 1, is divided by a predetermined voltage B.sup.+ and the resistors R1, R2 to be level-adjusted. ##EQU1##
The amplitude and level of the DC of the negative polarity pulse voltage Va can be adjusted according to the values of the resistors R1, R2. The waveform of the negative polarity pulse voltage Va divided by the resistors R1, R2 is corrected by the resistor R3 and the capacitor C4 of the electro-magnetic wave offset section 74. The negative polarity pulse voltage Va, the waveform of which has been corrected by the resistor R3 and the capacitor C4, is inputted to the lead wire 75 after the spike-like noise is eliminated by the resistor R4 and the capacitor C4.
The lead wire 75 induces only a predetermined voltage on the front face of the CRT 71 without inducing the electric current component of the inputted negative polarity pulse signal Va on the front face of the CRT 71. Accordingly, the radiated electric field wave emitted through the front face of the CRT 71 is offset by the negative polarity pulse voltage Va of the lead wire 75, the amplitude of which is the same thereas but the phase of which is opposite thereto.
However, such a conventional circuit for eliminating electronic wave in a video appliance has a disadvantage that it fails to completely eliminate the radiated electric field noise generated from the front face of the CRT 71. Also, whereas the scale and waveform of the noise of the radiated electric field generated from the front face of the CRT 71 are variable, no amplitude control means has yet been available to generate the negative polarity pulse voltage in conformity with the scale of the noise of the radiated electric field emitted from the front face of the CRT 71. Thus, the noise of the radiated electric field induced to the CRT 71 cannot be completely offset, thereby failing to satisfy the standard required by the Swedish Confederation of Professional Employees (TCO).